Exhaust gas discharged from an internal combustion engine of, for example, an automobile contains toxic components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Hitherto, three-way catalysts have been used for removing such toxic components for detoxifying the exhaust gas.
Such a three-way catalyst includes a noble metal (e.g., Pt, Pd, or Rh) serving as a catalytically active component; a material such as alumina, ceria, zirconia, or oxygen-occluding ceria-zirconia composite oxide, serving as a carrier; and a catalyst support made of a ceramic or metallic material and having a shape of honeycomb, plate, pellet, etc.
The regulation of automobile exhaust gas has been more strict recently, which has increased the demand and price of Pt or Rh, serving as a main catalytically active component of internal engine, exhaust gas purification catalysts. Particularly, a change in price of Rh provides great effects, and thus demand has arisen for replacement of Rh with Pd, which among noble metals is inexpensive. Under such circumstances, efforts have been made toward reduction of exhaust gas purification catalyst production cost by use of relatively inexpensive Pd as a catalytically active component, and various means therefor have been proposed (see, for example, Patent Documents 1, 2, and 3). Also, there has been proposed a catalyst containing aluminum borate as a carrier; specifically, a catalyst which exhibits improved gas diffusibility and is produced by causing a catalyst component to be supported on a powder compact containing a powdery material having hollow portions therein and covered with aluminum borate whiskers (see Patent Document 4).